It is known to use ceramic thermal barrier coatings to protect metallic parts that are exposed to hot combustion gas in a gas turbine engine. United States Patent Application Publication US 2009/0110954 A1 describes known thermal barrier coating systems which typically include a bond coat material deposited between the ceramic thermal barrier coating and the underlying metal substrate. It is also known that improved adherence of the thermal barrier coating can be achieved by providing a roughened surface on the bond coat material, such as by controlling the process parameters used to deposit the bond coat material. One such technique is described in United States Patent Application Publication US 2010/0092662 A1.